This invention relates generally to valves for use in controlling the flow of corrosive fluids, and more particularly to large dimension corrosion-resistant butterfly valves with pressurizeable seals. In the past, butterfly valves have typically been used under conditions where some leakage is acceptable. It has proven difficult to provide a positive annular seal about the disc or blade of such valves. Therefore, to minimize leakage, it is desirable to use pressure enhanced seals.
U.S. Pat. No. 1,813,126 to Shepphard discloses an early form of pressure-sealed butterfly valve. In such valve, an O-ring seal is seated in a cylindrical channel extending around the fluid passageway through the valve body. The valve disc or blade, mounted on a shaft journaled in the valve body, has a flat periphery or edge face. When the blade is closed, the seal is inflated to abut the periphery of the blade. Similar designs are disclosed in U.S. Pat. No. 2,705,016, to Saar, which further discloses offsetting the shaft from the seal, and U.S. Pat. No. 3,394,914 to Nagasato, which further discloses a split valve body construction.
Pressure-sealed butterfly valves utilizing other forms of seals are disclosed in U.S. Pat. No. 2,673,708 to Danks; U.S. Pat. No. 3,840,208 to Schudel et al; and U.S. Pat. No. 4,026,514 to Sumner et al.
The principal drawback of the foregoing designs is their inapplicability to the problem of providing valves, particularly large dimension butterfly valves, in conduits for corrosive fluids, such as pulp slurry or corrosive gases. While the Sheppard, Saar, Danks and Schudel et al patents all disclose large-dimension butterfly valves, only the Nagasato patent addresses, with reference to FIGS. 13-15, adaption of such a valve for corrosion resistance. It is apparent from the complexity and manner of construction of the foregoing valves, particularly the seats for the sealing members, that they were designed to be constructed of metal. However, most metals are unsuitable for exposure to highly corrosive fluids, unless coated with corrosion-resistant material. As noted by Nakasato, use of anti-corrosive coatings is not well-suited to medium and large valves, necessitating making the entire valve of metals that are suitable for use uncoated, such as titanium, hard lead, or high grade stainless steel. However, such metals are very heavy or very expensive, particularly for use in large dimension valves, such as those disclosed in the Saar and Schudel et al patents.
In an attempt to overcome these and other drawbacks, Ershigs, Inc. of Bellingham, Wash., has designed and fabricated large-diameter, fiberglass-reinforced plastic butterfly valves with titanium shafts and sleeves. The valve body provides a flat-surfaced cylindrical passageway and an O-ring seal is seated in the periphery of the valve blade. However, this design makes it very difficult to pressurize the seal. Also, the continued use of expensive titanium shafts and sleeves would preferably be avoided. Nothing in the prior art appears to suggest how to construct a fiberglass-reinforced plastic valve with a pressurizeable seal, or to avoid using titanium shafts and sleeves. Accordingly, there remains a need for a corrosion-resistant butterfly valve for application in large diameter passages for controlling the flow of highly corrosive fluids, and for a seal in such a valve which can be pressurized to provide, when desired, complete stoppage of fluid flow.